Automatic gas pressure control for electron beam apparatus



F TRQ4 UR 31243-9570 arch 29, 1966 BORING 3,243,570

AUTOMATIC GAS PRESSURE CONTROL FOR ELECTRON BEAM APPARATUS Filed A ril 50, 1963 5 Sheets-Sheet 1 fi/is At orney.

March 29, 1966 K. 1.. BORING 3,243,570

AUTOMATIC GAS PRESSURE CONTROL FOR ELECTRON BEAM APPARATUS Filed April 50. 1963 3 Sheets-Sheet 2 F g. J:

} .I I L 40 [271/677 6 on: Kenneth .1. fior/zzg, by 2 f M HAS A2; 260M723.

K. L. BORING 3 Sheets-Sheet 5 AUTOMATIC GAS PRESSURE CONTROL FOR ELECTRON BEAM APPARATUS Filed April 30.

March 29. 1966 [)7 vemfiar: Kenneth Lfior/zzg, by p m /7/5 Attorney.

United States Patent 3,243,570 AUTOMATIC GAS PRESSURE CONTROL FOR ELECTRON BEAM APPARATUS Kenneth L. Boring, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 30, 1963, Ser. No. 276,845 Claims. (Cl. 219-121) My invention relates to certain improvements in the electron beam irradiation apparatus of the gaseous beam type whereby the intensity and focus of the beam may be more effectively controlled, and in particular, to an improvement in the manner of controlling the gas pressure and thus controlling the electron beam produced by an electron beam apparatus of the type described in the copending United States Patent application of L. H. Stauffer, Serial No. 247,730, filed December 27, 1962, entitled Self-Focusing Electron Beam Apparatus, andassigned to the assignee of the present invention, now Patent No. 3,218,431.

The electron beam apparatus described in the above copending United States patent application is especially useful for welding, heating, and processing materials in controlled environments. The apparatus comprises a housing within which is positioned a hollow perforated cathode structure adapted to be operable in a low pressure ionizable gaseous medium and to be operated at a high negative potential relative to the housing, sufiicient to produce a body of ionizable gas or plasma within the cathode. The cathode is provided with an aperture through which an electron beam issues from the plasma and passes to a workpiece upon which it impinges and produces a desired efiect. Within a particular range of gas pressure and cathode-to-housing potential, interaction between the gaseous medium and high potential generates a finely focused or collimated electron beam having a current magnitude or beam intensity determined by the gas pressure and electrical potential. A control electrode structure may be positioned within the cathode and electrically insulated therefrom. The beam intensity is then controlled independently of beam focus by adjusting a low potential between the control electrode and cathode.

While the hereinabove described electron beam apparatus is satisfactory for welding, heating, and irradiation applications, certain improvements may be desirable to more effectively control the electron beam. In particular, it may be more convenient to provide a means for automatically stabilizing the beam intensity and focus during spurious variations in the gas pressure rather than by manually controlling the gas pressure. The gas pressure may vary due to liberation of gas or evaporation of material from the surface of the beam irradiated work piece and gas evolving from interior walls of the housing due to radiant heat emitted by the irradiated workpiece. Means for minimizing the effect of such undesired gas being liberated by the irradiated workpiece by manual means is described in the above copending application. Such corrective measures are based primarily on differential pumping to partially isolate the cathode region from the workpiece region. However, spurious variation of the gas pressure is more conveniently regulated by automatically controlling the gas pressure.

Therefore, one of the principal objects of my invention is to provide an automatic gas pressure control for an electron beam apparatus.

A further object of my invention is to effect such control by providing a control system for regulating the flow of gas through a valve as a function of selected electron beam characteristics.

The electron beam apparatus describedin the copending patent application has great utility in metal working 3,243,570- Patented Mar. 29, 1966 such as cutting, welding, brazing, and fusing materials wherein high rocessing temperatures in the order of 3000 C. are required. The low pressure of the gaseous medium within which the cathode is operable renders this electron beam apparatus especially useful in pressure conditions such as those existing around an orbiting space vehicle or within a vacuum chamber such as a space simulator.

Thus, a still further object of my invention is to provide a means for employing the subject electron beam apparatus in a vacuum environment.

Briefly stated and in accordance with my invention, I provide a circuit for developing an electrical signal which is a function of a selected electron beam characteristic such as the magnitude of the electron beam current or focus of the beam generated within the apparatus. Such signal is compared with a reference voltage representing a desired value of the selected electron beam characteristic. Variation from such desired value is employed to actuate an electromechanical transducer which adjusts a gas valve to control the pressure of the gaseous medium in the region of the cathode and thereby regulate the electron beam to its desired condition.

The subject electron beam apparatus may be utilized in a vacuum environment by containing the cathode within a flexible nonporous device such as a plastic container and inflating this device to the appropriate gas pressure which is required for beam mode operation.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIGURE 1 is an elevation view, partly in section, illustrating the combination of the subject'electron beam irradiation apparatus and a plurality of circuits for developing electrical signals which represent selected characteristics of the electron beam;

FIGURE 2 illustrates a circuit which converts the electrical signal to a mechanical motion for actuating the gas valve in accordance with my invention;

FIGURE 3 illustrates a first embodiment of the operation of the subject electron beam apparatus in a vacuum environment;

FIGURE 4 illustrates a second embodiment of the operation of the apparatus in a vacuum environment; and

FIGURE 5 illustrates a family of curves indicating the variation of the magnitude of beam current with various pressures of the gaseous medium maintained in the region of the cathode.

Referring particularly to the apparatus illustrated in FIGURE 1, there is shown a housing or enclosure designated as a whole by numeral 1. The housing is constructed of an electrically conductive and nonporous material such as metal. The interior of housing 1 may consist of a single chamber as illustrated in FIGURE 4, or a plurality of chambers. The particular apparatus illustrated in FIGURE 1 includes a first chamber 2 for containing the cathode structure and a second chamber 3 for containing the workpiece 4 to be irradiated by the electron beam emitted from the cathode. Chambers 2 and 3 are separated by a partitioning member 5 which may be made of the same material as housing 1. Partition member 5 is provided with an aperture 6 and is electrically insulated from housing 1 by means of an electrically nonconductive support ring 7. A suitable ionizable gas, such as argon or helium, is introduced at a low pressure to the interior of chamber 2 by way of passage means 8 which passes through a wall of lusing 1. Passage means 8 is connected to a gas supply (not shown) by means of a first gas flow valve 9 which regulates the rate of gas fiow into housing 1. A second passage means 10 passes through another wall of housing 1, and by virtue of its larger diameter, presents a low impedance exit for any gas generated by workpiece 4 while being irradiated by the electron beam and also aids in maintaining a desired gas pressure within chamber 2. Passage means 10 is connected to a suitable exhaust pumping system (not shown) by means of a second controllable valve 11. Thus, the gas pressure within chamber 2 is regulated by controlling the passage of gas through either or both of valves 9 and 11.

The electron source consists of a hollow perforated cathode structure 12 preferably in the form of a cylinder, although other shapes may be employed, with an aperture 13 in the center of a bottom end wall thereof wherefrom an electron beam is emitted by nonthermionic means in a manner more fully described in the above mentioned copending patent application. A control electrode structure (not shown) having an aperture aligned with cathode aperture 13 and aperture 6 may be positioned within the cathode to provide a means of controlling the beam intensity independent of beam focus.

A power supply (not shown) is connected to terminals 14, 15 and preferably supplies a relatively high adjustable direct current voltage to the cathode. Other suitable power sources, such as alternate half waves of an alternating current voltage, may also be employed. Thus, the cathode is operated at a high negative potential relative to housing 1 which functions as the anode. The voltage is applied to cathode 12 by means of conductor 16 and cathode stem 17 connected thereto. Cathode stem 17 comprises a hollow tubular electrically conductive member, preferably made of stainless steel that supports cathode 12 and positions it within housing 1. Cathode stem 17 is insulated from housing 1 by means of insulating bushing 18. The top end of cathode stem 17 may be enclosed or the interior of stem 17 filled with a suitable material to form a gas-tight seal between the interior and exterior of housing 1. A hollow tubular electrically conductive shield 19, concentric with cathode stem 17 and spaced therefrom, is positioned along the upper portion of stem 17 contained within housing 1. Shield 19 is in good electrical contact with housing 1 and prevents long-path discharge between cathode stem 17 and housing 1. The high voltage supplied at terminals 14, 15 is adjustable to 100 kilovolts or more. The preferred operating range is 10-25 kilovolts since X-ray hazards and voltage insulation problems are minimized at such lower voltages. The pressure of the gaseous medium is adjustable up to approximately 50 microns, the particular pressure necessary to maintain beam mode operation being dependent on the cathode voltage and particular gas employed. Interaction between the high negative cathodeto-housing potential and the ionizable gaseous medium contained within the housing produces a glowing body of plasma or ionized gas within the cathode structure. Electrons are emitted from the plasma body and pass through cathode aperture 13 in the direction of workpiece 4. The electron emission from cathode aperture 13 forms a well-collimated beam of electrons within particular narrow ranges of cathode-to-housing potential and gas pressure.

Variations of gas pressure within housing 1 cause variations in both beam focus and intensity. The magnitude of this effect may be appreciated by reference to FIGURE 5 which illustrates the variation of beam intensity with pressure of argon gas. Further, the inherent operation of the subject electron beam apparatus within the desired beam mode is possible only within narrow ranges of gas pressure for particular cathode voltages and a diffuse glow discharge occurs in operation above these pressure ranges. Spurious variation in the gas pressure is not accurately controllable by manual operation of the gas valves due to the inherent slower response time of the human operator. Even a relatively small change in gas pressure when not corrected has a pronounced effect on the electron beam and irradiated workpiece. FIGURE 5 indicates the magnitude of the etieet of a variation in gas pressure on the variation in beam intensity or current for a fixed cathode voltage. Thus, at a cathode voltage of 12 lrv., a 17 percent change in gas pressure (from 6 to 7 microns) produces a 200 percent change in beam current (from 5 to 15 milliamperes) as well as an appreciable defocusing of the beam. In general, it may be stated that in welding operations a change in gas pressure from that desired will result in an inferior weld. Thus, an increase in gas pressure at a constant cathode voltage tends to melt completely through the workpiece and also broaden the weld seam. In like manner, a decrease in gas pressure also broadens the weld seam and substantially decreases the power concentration 011 the workpiece, thereby preventing surficient penetration of the welding energy. Hence, an automatic means for maintaining more precise regulation of gas pressure in the region of the cathode is desirable for high quality metal working such as cutting, welding, and brazing.

A precise regulation of the electron beam is obtained by sensing particular characteristics of the electron beam and employing the signals obtained therefrom in a control system which regulates the gas flow valve 9 or 11 in a direction to maintain substantially constant beam conditions. In particular, it is noted from FIGURE 5 that beam current or intensity varies as a direct function of gas pressure. A second beam characteristic which may be utilized as a signal for the control system is beam focus. Within the particular narrow ranges of gas pressure and cathode voltage in which beam mode operation occurs, the most finely focused or collimated beam exists only at a single gas pressure for :a particular cathode voltage. A change in the gas pressure while maintaining constant cathode voltage defocuses or broadens the cross section of the beam. The gas pressure may thus be adjusted by sensing variations in the beam intensity or focus and deriving an electrical signal therefrom and employing this signal to control a gas valve in a direction to restore the beam to its original condition.

An electrical circuit for sensing change in beam focus includes potentiometer 20 connected to partition member 5 by means of electrical conductor 21 which passes through insulating bushing 22 disposed in a wall of hous- 1ng 1. In the presence of a change in gas pressure at constant cathode voltage, the electron beam defocuses and is partially intercepted by partition member 5 since aperture 6 is of size sufiicient to pass only a beam of selected cross section. The portion of the electron beam untercepted by partition member 5 causes electrons to flow to the positive terminal of potentiometer 20. Movable arm 23 on potentiometer 20 picks oil? a predetermined voltage drop across potentiometer 20 as an input signal to a servo control amplifier 24 which may be of a conventional electron or magnetic type, as illustrated in FIGURE 2. An electromechanical transducer 25 which may comprise a direct current motor and suitable gearmg is connected to the output of amplifier 24 and drives the valve shaft it? of valves 9 or 11. The control action 18 obtained in the following manner: During normal operating conditions of constant cathode supply voltage and gas pressure, there is no electron interception by partinon member 5 and therefore no resulting current flow through potentiometer 20. Thus, there is no output error signal from amplifier 24 and valve shaft 26 is not driven by transducer 25. During a spurious variation in gas pressure, the electron beam diverges or defocuscs and is partially intercepted by partition member 5 resulting in a current flow proportional to the degree of divergence of the electron beam. The magnitude of the resultant voltage drop across potentiometer deter-mines the magnitude of the input signal to amplifier 24 and thereby determines the amount of rotation of valve shaft 26 which adjusts gas pressure valves 9 or 11 in a direction to restore the initial beam condition. A spurious increase in gas pressure is controlled by a partial closing of valve 9 or a greater opening of valve 11. In like manner, a spurious decrease in gas pressure is controlled by a greater opening of valve 9 or a partial closing of valve 11.

An electrical circuit for sensing the beam current alone is provided by connecting a potentiometer 27 in series between positive voltage terminal 14 and housing 1. Movable arm 28 of potentiometer 27 picks off a proportional voltage drop across potentiometer 27 as an input signal to amplifier 24. A second input to amplifier 24 is provided from an electrical circuit comprising reference potentiometer 29 and a direct current voltage supply such as a battery 30 connected across potentiometer 29. This particular control circuit functions in the following manner: During normal operating conditions of constant voltage and gas pressure, the desired beam current flowing through potentiometer 27 provides an input signal to amplifier 24. Movable arm 31 of reference potentiometer 29 is adjusted to provide a voltage equal and opposite to the input signal from potentiometer 27. Thus, during steady state conditions there is no output from amplifier 24 and the gas valves remain unaffected. In the presence of a spurious increase in gas pressure, beam current increases and the greater voltage drop resulting across potentiometer 27 provides an output error signal from amplifier 24 which drives gas valve 11 to a more open position, thereby decreasing the gas pressure within chamber 2 and restoring the initial current. Alternatively, a spurious decrease in gas pressure develops a signal which drives the gas valve in the opposite direction.

A third embodiment of my automatic gas pressure control senses electron beam current by coupling a potentiometer 32 to workpiece 4 by means of electrical conductor 33, the conductor passing through insulating bushing 34 disposed in a wall of housing 1. Potentiometer 32 performs a similar function as potentiometer 27, it being understood that aperture 6 must be sufficiently large in this case to prevent interception of the electron beam by partitioning member 5. Alternatively, aperture 6 may be maintained at a size merely sufficient to pass a wellfocused electron beam therethrough and the input signal represented by the voltage picked off by movable arm 35 then represents a function of both variation in electron beam intensity and focus.

Potentiometers 20, 27, and 32 may be used singly or in combination to provide input signals to amplifier 24 which represent particular operating conditions. Thus, during the initial interval of beam formation, a shield may be placed between aperture 6 and workpiece 4 whereby the workpiece is shielded from the beam and potentiometer 27 employed to obtain the desired beam current and potentiometer 20 to obtain the desired beam focus. Upon obtaining the desired beam conditions, the shield is removed and potentiometers 27 and 20 may continue in operation, or, one or both may be removed and potentiometer 32 may be employed as the sensing element. Any desired combination of the sensing potentiometers may thus be employed to obtain a particular sensing function.

A gas pressure gauge which is suitable for measuring pressures in the range up to approximately 50 microns is conveniently employed as a further sensing element to provide an additional input signal to amplifier 24. Gauge 36 is connected through a wall of housing 1 to sense the gas pressure in the region of cathode 12. The desired gas pressure is determined from previous operations and the gauge is especially useful in presetting the desired operating pressure. Subsequently, the cathode power supply is connected to terminals 14, 15 and the cathode voltage is increased to a selected value which produces the desired beam conditions. The electrical signal from gauge 36 may be utilized in combination with any of the signals obtained from potentiometers 20, 27, and 32 in any well known manner, such as, by coupling the selected outputs to a conventional operational amplifier circuit commonly employed in analog computer technology. In one arrangement, the gauge signal is over-ridden by the potentiometer signal. In a second arrangement, the gauge signal is selectively removed from the control circuit at a predetermined level of potentiometer signal. Circuits embodying these, and other arrangements, are described in many publications including the following books published by McGraw-Hill Book Company, Analog Computation in Engineering Design by Rogers and Connolly, published 1960, and Analogue Computation, Volumes I-IV, by Fifer, published 1961.

A particular application of the subject electron beam apparatus in a vacuum environment such as a space simulator is illustrated in FIGURE 3. The electron beam apparatus is contained within a flexible nonporous container such as a plastic bag 37. A supply 38 of the gaseous medium which generates the plasma within cathode 12 is disposed to the exterior of plastic bag 37 and connected thereto by regulating valve 9 which may be manually operated or automatically controlled as hereto fore described. The interior of container 37 is thus provided with the gaseous medium in which the cathode op crates and this low pressure maintains container 37 inflated since the exterior of the container is a vacuum. An operator 39 positions the apparatus at the desired point at which a weld or other electron irradiation process is to be performed, the operator being equipped with an air breathing apparatus. Container 37 may be anchored to the floor of the space simulator by a suitable means which does not necessarily have to provide a gas-tight seal.

A more compact vacuum environment application of the electron beam apparatus is illustrated in FIGURE 4. Here, operator 39 is positioned outside of plastic bag 37 and the bag is of such small size as to be easily moved in reference to workpiece 4. The electron beam apparatus in this particular configuration includes no side wall for housing 1. Cathode 12, end wall 1, cathode stem 17, and bushing 18 are supported by inflated plastic bag 37 which has strut members running along the outer edges of inflated bag 37 from housing end wall 1 to the base 40 upon which bag 37 is supported. An electrical conductor 41 is connected between workpiece 4 and one of the struts which are made of an electrically conductive material to provide a return path to the positive terminal of the power supply. This configuration permits visual inspection of the entire irradiation operation since container 37 may be made of a transparent plastic material. Although the electron beam apparatus illustrated in FIG- URE 3 is semi-portable, the apparatus in FIGURE 4 is completely portable and finds use in applications such as repairing the outer surface of an orbiting space vehicle. In such application, a member in the form of a handle may be attached to end wall 1 and be provided with a suitable mechanism which permits manual or automatic control of the cathode voltage and gas pressure. The operator employs the handle to place the electron beam apparatus, including strut-supported container 37, in communication with workpiece 4 on the surface 40 of the space vehicle to be repaired. A clearance between the bottom of container 37 and surface 40 of the space vehicle may be tolerated with the attendant requirement for supplying a sufficient amount of gas to maintain the required gas pressure in the region of cathode 12. A substantially gas-tight seal does, however, permit use of a smaller gas supply 38 since such seal minimizes the es cape of the gaseous medium from the interior of container 37 to the vacuum on the exterior thereof. An outlet for blowing out the undesired products of outgassing from the irradiated workpiece may be provided in container 37. In the particular application of FIGURE 4, the operator should be provided with a suit made of suitable radiation-resistant material since the absence of metallic housing 1 side walls and use of plastic material 37 permits the radiation of X -rays, which are generated at the cathode, to the exterior of container 37. Alternatively, a heat shield may be supported within container 37 and disposed around the cathode and workpiece to act as an X-ray shield and a heat shield to prevent melting of the plastic material 37.

From the foregoing description, it can be appreciated that my invention makes available a control system for automatically controlling the gas pressure within an electron beam apparatus comprising a perforated hollow cathode operable at a relatively high negative voltage and disposed in a relatively low pressure gaseous medium. The automatic gas pressure control stabilizes the electron beam at a relatively constant beam intensity and focus. The low pressure gaseous medium renders the apparatus especially useful in vacuum environments.

Having described a number of embodiments of a new and improved control circuit for automatically controlling the gas pressure within a particular electron beam apparatus, it is believed obvious that modification and variation of my invention are possible in the light of the above teachings. Thus, both gas valves may be controlled either alternately or simultaneously provided the proper directions are employed in driving the valve shafts which control the valves. A second servo control amplifier and electromechanical transducer may be employed in driving the second control gas valve. Parameters other than electron beam current and focus may also be controlled by a simple modification of my invention. Thus, electron beam power may be regulated by providing a suitable circuit, such as a conventional operational amplifier circuit or direct current wattmeter with provision for an electrical analog output signal, for combining the beam current signal with a signal which is a function of the cathode voltage and obtaining the product thereof. Alternatively, the temperature of the workpiece being processed can be measured by conventional pyrometric methods such as use of a conventional optical pryometer or thermocouple with appropriate amplifier circuitry and an electrical signal derived therefrom employed as an alternative or additional input to the servo amplifier, thereby regulating the workpiece temperature. A preferred embodiment employing a temperature signal operates in a manner whereby a gas pressure signal initially regulates the gas valve to obtain the desired gas pressure, then a beam current signal overrides the pressure signal to obtain the desired beam current, and finally a workpiece temperature signal overrides both the pressure and current signals to regulate workpiece temperature. It is, therefore to be understood that changes may be made in the particular embodiments as described which are Within the full intended scope of the invention and defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufliciently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, with means for controlling the pressure of the gaseous medium within said enclosure, said means being responsive to variations in selected gas pressure related characteristics of the electron beam.

2. The combination of a cathode comprising a hollow perforated structure positioned in an ionizable gaseous medium and operated at a high negative potential relative to the enclosure of said medium sufficient to produce a plasma within the cathode, said cathode having an aperture through which an electron beam issues from the plasma, with means for sensing selected gas pressure related characteristics of the electron beam, and

means responsive to said sensing means for controlling the pressure of the gaseous medium within the enclosure thereby regulating the beam focus and beam intensity.

3. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, said cathode operable at a potential negative relative to said enclosure and sutficiently high to produce a plasma within said cathode, said cathode having an aperture through which an electron beam issues from the plasma, with resistance means for developing electrical signals which are a function of selected gas pressure related characteristics of the electron beam, and

means responsive to said electrical signals for controlling the pressure of the gaseous medium Within said enclosure thereby regulating the beam focus and beam intensity.

4. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for operating said cathode at a potential negative relative to said enclosure and sutficiently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, with a control electrode positioned within said cathode and electrically insulated therefrom, said control electrode having an aperture aligned with said cathode aperture whereby the intensity of the beam may be varied by varying a low potential between said control electrode and cathode,

resistance means for developing electrical signals which are a function of selected gas pressure related characteristics of the electron beam, and

means responsive to said electrical signals for control ling the pressure of the gaseous medium within the enclosure thereby regulating the beam focus and beam intensity.

5. In an electron beam welding apparatus comprising a housing,

means for defining a plurality of enclosures within said housing,

a cathode comprising a hollow perforated structure, said cathode disposed within a first of said enclosures, said first enclosure containing an ionizable gaseous medium, said cathode operable at a high negative potential relative to the housing sufiicient to produce a plasma within said cathode, said cathode and said means each having an aperture, the apertures being aligned with each other whereby an electron beam issuing from the plasma passes through. said apertures into another of said enclosures, said other enclosure being adapted to support a workpiece to be welded by said electron beam,

21 gas flow valve for adjusting the pressure of the gaseous medium within said enclosures,

an electrical circuit comprising a variable resistance for developing electrical signals which are a function of selected gas pressure related characteristics of the electron beam, and

an electromechanical transducer for converting the electrical signals to mechanical motion which regulates said valve there-by controlling the beam focus and beam current.

6. The combination set forth in claim 5 wherein an additional electrical signal proportional to the gas pressure within said first enclosure is supplied to said transducer.

7. In an electron beam welding apparatus comprising a housing,

partitioning means for defining a plurality of enclosures Within said housing,

a cathode comprising a hollow perforated structure, said cathode disposed within a first of said enclosures, said first enclosure containing a low pressure ionizable gaseous medium, means for maintaining said cathode at a potential negative relative to the housing and sufficiently high to produce a plasma within the cathode structure, said cathode and said pahtitioning means each having an aperture, the apertures being aligned with each other whereby an electron beam issuing from the plasma passes through said apertures into another of said enclosures, said other enclosure being adapted to support a workpiece to be welded by said electron beam,

a gas flow valve for controlling the pressure of the gaseous [medium within said first enclosure,

a first potentiometer having one end connected to said partitioning means,

a servo control amplifier, the movable arm of said first potentiometer connected as a first input to said amplifier, a second potentiometer providing a reference voltage as a second input to said amplifier, said reference voltage determining desired gas pressure related characteristics of the beam for the particular welding operation, and

an electromechanical transducer, output of said amplifier connected to the input of said transducer, output of said transducer mechanically coupled to said valve whereby variations in electron beam focus produce signals at the output of said amplifier and thereby actuate said transducer which operates said valve to vary the flow of gaseous medium thereby regulating the beam focus and beam intensity.

8. In an electron beam welding apparatus comprising a housing,

means for defining a plurality of enclosures within said housing,

a cathode comprising a hollow perforated structure,

said cathode disposed within a first of said enclosures, said housing containing a low pressure ionizable gaseous medium, said cathode operable at a high negative potential relative to the housing sufficient to produce a plasma within said cathode, said cathode and said means each having an aperture, the apertures being aligned with each other whereby an electron beam issuing from the plasma passes through said apertures into another of said enclosures, said other enclosure being adapted to sup port a workpiece to be welded by said electron beam,

a gas flow valve for controlling the pressure of the gaseous medium within said first enclosure,

a first potentiometer having one end connected to said workpiece,

a servo control amplifier, the movable arm of said first potentiometer connected as a first input to said amplifier, a second potentiometer providing a reference voltage as a second input to said amplifier, said reference voltage determining the desired beam intensity for the particular welding operation, and

an electromechanical transducer, output of said amplifier connected to the input of said transducer, output of said transducer mechanically coupled to said valve whereby variations in electron beam intensity produce signals at the output of said amplifier and thereby actuate said transducer which operates said valve to vary the flow of gaseous medium thereby regulating the beam focus and beam intensity.

9. In an electron beam welding apparatus comprising a housing,

means for defining a plurality of enclosures within said housing,

a cathode comprising a hollow perforated structure,

said cathode disposed within a first of said enclosures and operable in a low pressure ionizable gaseous medium, a cathode power supply for maintaining said cathode at a potential negative relative to the housing and sufficiently high to produce a plasma within the cathode structure, said cathode and said defining means each having an aperture, the apertures being aligned with each other whereby an electron beam issuing from the plasma passes through said apertures into another of said enclosures, said other enclosure being adapted to support a workpiece to be welded by said electron beam,

a gas flow valve for controlling the pressure of the gaseous medium within said first enclosure,

a first potentiometer connected to said housing and the positive terminal of said cathode power supply,

a servo control amplifier, the movable arm of said first potentiometer connected as a first input to said amplifier, a second potentiometer providing a reference voltage as a second input to said amplifier, said reference voltage determining the desired beam intensity for the particular welding operation, and

an electromechanical transducer, output of said amplifier connected to the input of said transducer, output of said transducer mechanically coupled to said valve whereby variations in electron beam intensity produce signals at the output of said amplifier and thereby actuate said transducer which operates said valve to vary the flow of gaseous medium thereby regulating the beam focus and beam intensity.

10. In an electron beam welding apparatus comprising a housing containing an ionizable gaseous medium,

partitioning means for defining a plurality of enclosures within said housing, a cathode positioned within a first of said enclosures and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said housing sufiiciently high to produce a plasma Within said cathode structure, said cathode and said partitioning means each having an aperture, the apertures being aligned with each other whereby an electron beam issuing from the plasma passes through said apertures into another of said enclosures, said other enclosure being adapted to support a workpiece to be welded by said electron beam,

a gas flow valve for controlling the pressure of the gaseous medium within said first enclosure,

a first potentiometer having one end connected to said partitioning means,

a second potentiometer connected to said housing and the positive terminal of said potential means,

a servo control amplifier, the movable arm of said first potentiometer connected as a first input to said amplifier, the movable arm of said second potentiometer connected as a second input to said amplifier, a third potentiometer providing a reference voltage as a third input to said amplifier, said reference voltage determining desired gas pressure related characteristics of the beam for the particular welding operation, and

an electromechanical transducer, output of said amplifier connected to the input of said transducer, output of said transducer mechanically coupled to said valve whereby variations in electron beam intensity and focus produce signals at the output of said amplifier and thereby actuate said transducer which operates said valve to vary the flow of gaseous medium thereby regulating the beam focus and beam intensity.

11. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufficiently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, and

means responsive to the magnitude of electron beam current for automatically controlling the pressure of the gaseous medium within said enclosure.

12. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufiiciently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, and

means responsive to the magnitude of electron beam current and gas pressure for automatically controlling the pressure of the gaseous medium within said enclosure.

13. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufiiciently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, and

means responsive to gas pressure and the temperature of a work piece being irradiated by the electron beam for automatically controlling the pressure of the gaseous medium within said enclosure.

14. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufficiently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, and

means for automatically controlling the pressure of the gaseous medium within said enclosure wherein said pressure controlling means is initially responsive to gas pressure and subsequently responsive to the magnitude of electron beam current and finally responsive to the temperature of a work piece being irradiated by the electron beam.

15. The combination of an enclosure containing an ionizable gaseous medium, a cathode positioned Within said enclosure and comprising a hollow perforated structure, means for maintaining said cathode at a potential negative relative to said enclosure and sufliciently high to produce a plasma within the cathode structure, said cathode having an aperture through which an electron beam issues from the plasma, and

means responsive to electron beam power for automatically controlling the pressure of the gaseous medium within said enclosure.

References Cited by the Examiner UNITED STATES PATENTS 3,054,896 9/1962 Jones et al 250-495 3,109,136 10/1963 Asamaki 324-33 3,112,391 11/1963 Sciaky 219-124 RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner. 

1. THE COMBINATION OF AN ENCLOSURE CONTAINING AN IONIZABLE GASEOUS MEDIUM, A CATHODE POSITIONED WITHIN SAID ENCLOSURE AND COMPRISING A HOLLOW PERFORATED STRUCTURE, MEANS FOR MAINTAINING SAID CATHODE AT A POTENTIAL NEGATIVE RELATIVE TO SAID ENCLOSURE AND SUFFICIENTLY HIGH TO PRODUCE A PLASMA WITHIN THE CATHODE STRUCTURE, SAID CATHODE HAVING AN APERTURE THROUGH WHICH AN ELECTRON BEAM ISSUES FROM THE PLASMA, WITH MEANS FOR CONTROLLING THE PRESSURE OF THE GASEOUS MEDIUM WITHIN SAID ENCLOSURE, SAID MEANS BEING RESPONSIVE TO VARIATIONS IN SELECTED GAS PRESSURE RELATED CHARACTERISTICS OF THE ELECTRON BEAM. 